Secondary ion mass spectrometry (SIMS) is a widely-used surface and thin film analytical technique that finds wide application in the semiconductor industry, in geochemistry and materials research, and other technical areas. Over 500 commercial instruments exist world-wide. The technique generates an analytical signal by bombarding a sample with an energetic ion beam (the “primary” ion beam) that “sputters” atoms from the sample surface. Each impact of a 5-15 keV primary ion ejects a small number of atoms from the target surface. A fraction of the ejected atoms are ionized upon ejection and these “secondary” ions can be accelerated into a mass spectrometer and mass-analyzed to yield information about the chemical and isotopic make-up of the sample.
The efficiency of secondary ion formation can be increased by using chemically active primary ion species which are implanted in the target surface and alter its surface chemistry: electronegative primary ion species such as oxygen are used to enhance positive secondary ion yields, and electropositive primary ion species such as cesium ions are used to enhance negative ion yields (i.e., secondary negative ions of electronegative species).
The SIMS technique provides a unique combination of extremely high sensitivity for almost all elements from hydrogen to uranium and above (e.g., detection limit down to ppb level for many elements), high lateral resolution imaging (e.g., down to 50 nm currently), and a very low background that allows high dynamic range (e.g., more than 5 decades). This technique is “destructive” by its nature, since it involves sputtering of material to generate an ion signal. It can be applied to any type of material (insulators, semiconductors, metals) that can stay under vacuum.
One major strength of the SIMS technique is that it embodies a microanalytical method. The primary ion beam can be focused to a tiny spot so that chemical analysis can be performed on extremely tiny areas; alternatively, by rastering the focused beam over a sample surface while monitoring ion signals, chemical and isotopic images of the sample surface can be produced with excellent spatial resolution.
At present, the epitome of imaging performance occurs in an instrument called NanoSIMS (manufactured by Cameca, Paris, France), which has a present cost of approximately $4 million. This instrument has precisely designed primary ion optics intended to focus the primary ion beams to the smallest possible spot on the sample surface. The specified minimum beam size with the factory ion source is 50 nm, typically obtained with a beam current at the sample of ˜0.25 picoamps (pA).
The factory ion source design of a NanoSIMS instrument is schematically illustrated in FIG. 1A. The source 1 is fabricated completely of metal—mainly molybdenum, but with part of the ionizer section 7 being tungsten. The source 1 includes a mounting post 2, a heated molybdenum reservoir body 3 supported by the mounting post 2 and including a reservoir cavity 3A arranged to hold a cesium salt (e.g., cesium carbonate), a molybdenum narrow tube assembly 5 (including a wide base portion 6 serving as a portion of the reservoir), and a strongly-heated ionizer section 7 arranged to receive cesium carbonate vapor from the reservoir via the narrow tube. The narrow tube assembly 5 feeds cesium carbonate vapor from the heated reservoir body 3 to the strongly-heated ionizer section 7 and also provides a degree of thermal isolation between the reservoir body 3 and the ionizer section 7. An outer edge portion of the reservoir body 3 includes a beveled surface 3B.A bounding surface 6A of the wide base portion 6 of the narrow tube assembly 5 is arranged to abut the beveled surface 3B of the reservoir body 3. The reservoir body 3 is externally threaded and is arranged to receive an internally threaded sealing cap screw 4 that fits around the wide base portion 6 to form a swage-type seal. Sealing between the molybdenum wide base portion 6 and the molybdenum reservoir body 3 is a crucial issue for this ion source 1, since leakage causes poor performance of the electron impact heating system and ultimately causes noisy images. The swage-type seal between the two molybdenum reservoir portions 3, 6 utilized with the factory ion source 1 requires close control of the sealing force and is not designed to be demountable, so the ion source 1 cannot be reused.
A detailed view of the ionizer section of a NanoSIMS factory source is shown in FIG. 1B. A tip 10 of the ionizer section 7 serves as an electrode and defines an outlet aperture 9. A flat tungsten ionizer plate 8 is arranged in a widened cavity 5A between the narrow tube 5 (at bottom) and the outlet aperture 9. The aperture 9 typically has a diameter of about 0.5 mm (500 μm), and the tungsten ionizer plate 8 is typically spaced a distance of about 0.2 mm (200 μm) apart from an internal surface 11 of the tip 10 that serves as an electrode and that defines the aperture 9.
The intended (or design objective) operation of the ionizer section 7 is shown in FIG. 10, with further reference to structures depicted in FIG. 1A arranged upstream of the ionizer section 7. The reservoir body 3 is heated to cause cesium carbonate vapor to diffuse up the narrow tube 5 and decompose as the vapor reaches the strongly-heated ionizer section 7 (e.g., which is heated to about 1200° C.) where the vapor flows onto the flat tungsten ionizer plate 8. The ionizer section 7 is strongly heated (e.g., by a combination of electron bombardment and radiative heating from the electron emitting filament) and cesium atoms that impact the tungsten ionizer plate 8 evaporate almost 100% as positive ions. The source is held at high potential (+8 kV in the NanoSIMS) very close to a grounded extraction plate (not shown) and the cesium ions are extracted by the high electric field penetrating through the 500 μm aperture 9 in the electrode tip 10 around the ionizer plate 8. As shown in FIG. 10, this shaped electric field is designed to electrostatically accelerate ions and draw the ions into a small “crossover” that forms the ion-optical “object” for the focusing optics of the primary ion column to focus to a demagnified image at the sample (such as the 50 nm diameter factory specification for the NanoSIMS).
In practice, actual operation of the ionizer section differs from the intended operation schematically illustrated in FIG. 10. FIG. 1D illustrates the practical operation of the foregoing ionizer section 7. In practice, it is impossible to heat only the ionizer plate 8; instead, the entire ionizer head is heated and cesium ions are formed on (and extracted from) all surfaces throughout the ionizer volume. Arguably, cesium ions can be formed in, and extracted from a region 500 μm in diameter and 200 μm deep. This makes for a more diffuse ion-optical object, and this in turn results in the focused image at the sample being limited to the factory specification of 50 nm diameter. Compared to the design objective schematically illustrated in FIG. 1C, in practical operation the initial ion beam crossover is significantly compromised.
The factory ion source 1 shown in FIG. 1A is typically replaced one to several times per year (e.g., upon exhaustion of cesium salt source material), with the frequency of replacement depending on use of a NanoSIMS instrument. Such “disposable” ion sources cost about $3000 for each replacement.
An alternative ionizer design was developed at Arizona State University for use with an early version Cameca SIMS instrument (i.e., not the NanoSIMS instrument) around the year 2000. In one version, a ⅛″ outside diameter, 1/16″ inside diameter alumina tube, approximately 3″ long is used. One end of the tube is sealed with a commercial alumina cement plug and a fine hole or orifice (e.g., 0.010″ or 250 μm in diameter) is drilled through the cement plug. A quantity (approximately 0.15 g) of cesium carbonate is loaded into the other end of the tube, which is sealed with an alumina cap cemented in place with alumina cement. The end of the tube with the fine orifice is inserted into a resistance heater including heating elements and heated to approximately 1200° C. The Cs2CO3 charge is heated by heat conduction along the tube and vaporizes either as Cs2CO3 or after decomposition to Cs2O; the resulting vapor then effuses out of the orifice. At the high temperature in the orifice, the vapor dissociates to atomic cesium. Almost every cesium atom traversing the orifice makes multiple collisions with the heated alumina surface and has a very high probability of being thermally surface-ionized. The ionizer orifice produces a high flux density of cesium atoms through a tiny central area which can be accurately aligned with the primary ion column of the secondary ion mass spectrometer. Moreover, as compared to conventional ionizers fabricated out of expensive tungsten metal, the use of alumina (in particular alumina cement) means that the heat-resistant ionizer portion of the source is very inexpensive to fabricate because the alumina cement can be very easily drilled before heat-setting, or the cement plug can be formed around a fine wire insert which is later removed after the cement has set. The early version Cameca SIMS instrument with the ionizer section outlined above did not have a primary ion column capable of focusing the ion beam to an extremely fine spot; however, it was demonstrated that the total ion current was competitive with other ion sources of the era.
A graphite-based variant of the above-described alumina-based orifice ionizer section was developed at Arizona State University and has been in use at such institution since about 2001. The design of the graphite-based ionizer section 17 is shown in FIG. 2. Such ionizer section 17 includes a channel or orifice 29 fabricated in a graphite plug 20 that is screwed into a molybdenum reservoir tube 15 via threads 23 proximate to an end 15′ of the tube 15, with the tube 15 and plug 20 being heated by a resistance heater 12 including heating elements 13 arranged external to the molybdenum tube 15 and graphite plug 20. The molybdenum tube 15 is internally threaded and is arranged to receive external threads of the graphite plug 20. As shown in FIG. 2, the channel or orifice 29 has a diameter of 0.125 mm (125 μm), and the end surface 21 of the plug 20 is substantially flush with an end surface 28 of the reservoir tube 15.
Use of graphite confers certain benefits. Graphite is highly refractory so that it withstands the high temperature needed for surface ionization. Yet unlike refractory metals, graphite is soft and amenable to drilling with a fragile 0.005″ (125 micron) diameter drill. The softness of graphite also allows facile sealing of the drilled graphite insert to the metal reservoir tube. In FIG. 2, a beveled base 22 of the graphite plug 20 is forced into a sharp metal edge 16 of the tube 15, thereby cutting into the graphite material of the graphite plug 20 and providing a vapor seal. The surface work function of graphite is ˜4.5 electron-volts, comparable to tungsten and higher than the ionization potential of cesium (3.9 electron-volts), which ensures almost 100% ionization efficiency for cesium on the heated graphite surface.
The art continues to seek cesium ion sources for use with SIMS instruments that are capable of providing improved performance and reduced cost. Aspects of this disclosure address shortcomings associated with conventional systems and methods.